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\bibitem{FuncNonFunctional} \emph{http://en.wikipedia.org/wiki/Functional\_requirement}.
\bibitem{apuntesClase} Garví E.,  Holgado J. A.,  Rodríguez M.L., \emph{Software Engineering II. Classe notes. Lesson 4: 
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\bibitem{softwareengineering} Douglas Bell, \emph{Software Engineering, A programming Approach, Third Edition,} Pearson, 2000.